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Andrew S Weller

Publications and source records attributed to Andrew S Weller.

At least 19 recordsLinked to original sources
PubMed2006

The role of halogenated carborane monoanions in olefin hydrogenation catalysed by cationic iridium phosphine complexes.

Iridium hydridophosphine complexes of general formula [Ir(PR3)2H2(anion)](PR3= PPh3, PMe2Ph; anion =[1-closo-CB(11)H(6)Cl(6)]-, [1-closo-CB(11)H(6)I(6)]-, [BAr(F)4]-) have been prepared by hydrogenation of cyclooctadiene precursor complexes. Solid-state structures of selected examples of these complexes reveal intimate contacts between the carborane anion and cation, with the anion binding through two lower-hemisphere halogen ligands. In CD2Cl2 solution the very weakly coordinating anions [1-closo-CB(11)H(6)Cl(6)]- and [BAr(F)4]- are suggested to favour the formation of solvent complexes such as [Ir(PR3)2H2(solvent)n][anion], while the [1-closo-CB(11)H(6)I(6)]- anion forms a tightly bound complex with the cationic iridium fragment. Calculated DeltaG values for anion reorganisation in d8-toluene reflect this difference in interaction between the anions and cation. With the bulky anion [1-closo-CB(11)Me(5)I(6)]- different complexes are formed: Ir(PPh3)H2(1-closo-HCB(11)Me(5)I(6)) and [(PPh3)3Ir(H2)H2][1-closo-HCB(11)Me(5)I(6)] which have been characterised spectroscopically. Diffusion measurements in CD2Cl2 are also consistent with larger, solvent coordinated, complexes for the more weakly coordinating anions and a tighter interaction between anion and cation for [1-closo-CB(11)H(6)I(6)]-. All the complexes show some ion-paring in solution. Comparison with data previously reported for the [1-closo-CB(11)H(6)Br(6)]- anion shows that this anion--as expected--fits between [1-closo-CB(11)H(6)Cl(6)]- and [1-closo-CB(11)H(6)I(6)]- in terms of coordinating ability. Although not coordinating, the large [1-closo-CB(11)H(6)Cl(6)]- and [BAr(F)4)]- anions do provide some stabilisation towards the metal centre, as decomposition to the hydride bridged dimer [Ir2(PPh3)4H5]+ is retarded. This is in contrast to the [PF6]- salt where decomposition is immediate. As expected, complexes with the smaller phosphine PMe2Ph form tighter interactions with the carborane anions. These observations on the interaction between anion and cation in solution are reflected in benchmark hydrogenation studies that show a significant attenuation in rate of hydrogenation of cyclohexane on using the [1-closo-CB(11)H(6)I(6)]- anion or complexes with the PMe2Ph phosphine. We also comment on the reusability of the catalysts and their tolerance to water and oxygen impurities. Overall the catalyst with the [1-closo-CB(11)H(6)Br(6)]- anion shows the best combination of rate of hydrogenation, reusability and tolerance to impurities.

Journal Article↗
PubMed2006

High hydride count rhodium octahedra, [Rh6(PR3)6H12][BArF4]2: synthesis, structures, and reversible hydrogen uptake under mild conditions.

A new class of transition metal cluster is described, [Rh(6)(PR(3))(6)H(12)][BAr(F)(4)](2) (R = (i)Pr (1a), Cy (2a); BAr(F)(4) = [B{C(6)H(3)(CF(3))(2)}(4)](-)). These clusters are unique in that they have structures exactly like those of early transition metal clusters with edge-bridging pi-donor ligands rather than the structures expected for late transition metal clusters with pi-acceptor ligands. The solid-state structures of 1a and 2a have been determined, and the 12 hydride ligands bridge each Rh-Rh edge of a regular octahedron. Pulsed gradient spin-echo NMR experiments show that the clusters remain intact in solution, having calculated hydrodynamic radii of 9.5(3) A for 1a and 10.7(2) A for 2a, and the formulation of 1a and 2a was unambiguously confirmed by ESI mass spectrometry. Both 1a and 2a take up two molecules of H(2) to afford the cluster species [Rh(6)(P(i)Pr(3))(6)H(16)][BAr(F)(4)](2) (1b) and [Rh(6)(PCy(3))(6)H(16)][BAr(F)(4)](2) (2b), respectively, as characterized by NMR spectroscopy, ESI-MS, and, for 2b, X-ray crystallography using the [1-H-CB(11)Me(11)](-) salt. The hydride ligands were not located by X-ray crystallography, but (1)H NMR spectroscopy showed a 15:1 ratio of hydride ligands, suggesting an interstitial hydride ligand. Addition of H(2) is reversible: placing 1b and 2b under vacuum regenerates 1a and 2a. DFT calculations on [Rh(6)(PH(3))(6)H(x)()](2+) (x = 12, 16) support the structural assignments and also show a molecular orbital structure that has 20 orbitals involved with cluster bonding. Cluster formation has been monitored by (31)P{(1)H} and (1)H NMR spectroscopy, and mechanisms involving heterolytic H(2) cleavage and elimination of [HP(i)Pr(3)](+) or the formation of trimetallic intermediates are discussed.

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PubMed2005

Dihydrogen complexes of rhodium: [RhH2(H2)x (PR3)2]+ (R = Cy, iPr; x = 1, 2).

Addition of H2 (4 atm at 298 K) to [Rh(nbd)(PR3)2][BAr(F)4] [R = Cy, iPr] affords Rh(III) dihydride/dihydrogen complexes. For R = Cy, complex 1a results, which has been shown by low-temperature NMR experiments to be the bis-dihydrogen/bis-hydride complex [Rh(H)2(eta2-H2)2(PCy3)2][BAr(F)4]. An X-ray diffraction study on 1a confirmed the {Rh(PCy3)2} core structure, but due to a poor data set, the hydrogen ligands were not located. DFT calculations at the B3LYP/DZVP level support the formulation as a Rh(III) dihydride/dihydrogen complex with cis hydride ligands. For R = iPr, the equivalent species, [Rh(H)2(eta2-H2)2(P iPr3)2][BAr(F)4] 2a, is formed, along with another complex that was spectroscopically identified as the mono-dihydrogen, bis-hydride solvent complex [Rh(H)2(eta2-H2)(CD2Cl2)(P iPr3)2][BAr(F)4] 2b. The analogous complex with PCy3 ligands, [Rh(H)2(eta2-H2)(CD2Cl2)(PCy3)2][BAr(F)4] 1b, can be observed by reducing the H2 pressure to 2 atm (at 298 K). Under vacuum, the dihydrogen ligands are lost in these complexes to form the spectroscopically characterized species, tentatively identified as the bis hydrides [Rh(H)2(L)2(PR3)2][BAr(F)4] (1c R = Cy; 2c R = iPr; L = CD2Cl2 or agostic interaction). Exposure of 1c or 2c to a H2 atmosphere regenerates the dihydrogen/bis-hydride complexes, while adding acetonitrile affords the bis-hydride MeCN adduct complexes [Rh(H)2(NCMe)2(PR3)2][BAr(F)4]. The dihydrogen complexes lose [HPR3][BAr(F)4] at or just above ambient temperature, suggested to be by heterolytic splitting of coordinated H2, to ultimately afford the dicationic cluster compounds of the type [Rh6(PR3)6(mu-H)12][BAr(F)4]2 in moderate yield.

Journal Article↗
PubMed2004

Transition metal complexes of the chelating phosphine borane ligand Ph2PCH2Ph2P.BH3.

Chromium and ruthenium complexes of the chelating phosphine borane H(3)B.dppm are reported. Addition of H(3)B.dppm to [Cr(CO)(4)(nbd)](nbd = norbornadiene) affords [Cr(CO)(4)(eta1-H(3)B.dppm)] in which the borane is linked to the metal through a single B-H-Cr interaction. Addition of H(3)B.dppm to [CpRu(PR(3))(NCMe)(2)](+)(Cp =eta5)-C(5)H(5)) results in [CpRu(PR(3))(eta1-H(3)B.dppm)][PF(6)](R = Me, OMe) which also show a single B-H-Ru interaction. Reaction with [CpRu(NCMe)(3)](+) only resulted in a mixture of products. In contrast, with [Cp*Ru(NCMe)(3)](+)(Cp*=eta5)-C(5)Me(5)) a single product is isolated in high yield: [Cp*Ru(eta2-H(3)B.dppm)][PF(6)]. This complex shows two B-H-Ru interactions. Reaction with L = PMe(3) or CO breaks one of these and the complexes [Cp*Ru(L)(eta1-H(3)B.dppm)][PF(6)] are formed in good yield. With L = MeCN an equilibrium is established between [Cp*Ru(eta2-H(3)B.dppm)][PF(6)] and the acetonitrile adduct. [Cp*Ru (eta2-H(3)B.dppm)][PF(6)] can be considered as being "operationally unsaturated", effectively acting as a source of 16-electron [Cp*Ru (eta1-H(3)B.dppm)][PF(6)]. All the new compounds (apart from the CO and MeCN adducts) have been characterised by X-ray crystallography. The solid-state structure of H(3)B.dppm is also reported.

Journal Article↗
PubMed2004

[(iPr3P)6Rh6H12]2+: a high-hydride content octahedron that bridges the gap between late and early transition metal clusters.

Treatment of [(iPr3P)2Rh(nbd)][Y] {nbd = norbornadiene, Y = B{3,5-(CF3)2C6H3}4- [B(ArF)4] or 1-H-closo-CB11Me11-} with H2 (ca. 4 atm) results in the isolation, in moderate yield, of the octahedral cluster complex [(iPr3P)6Rh6H12][Y]2 1. The cluster (for both anions) has been characterized by NMR, mass spectroscopy, and X-ray crystallography. These show 1 to have 12 edge-bridging hydrogen atoms, and the structure bears more resemblance to clusters of the early transition metals with pi-donor ligands than those of the late transition metals with pi-acceptor ligands. Intermediate complexes on the route to 1, namely, the nonclassical dihydrogen complexes [(iPr3P)2Rh(H)2(eta2-H2)x][B(ArF)4] (x = 1 or 2), have been observed spectroscopically. The high hydride content of 1 makes it a possible model for nanocluster colloidal Rh(0) catalysts that are used in olefin and arene hydrogenation.

Journal Article↗
PubMed2004

Silver-phosphine complexes of the highly methylated carborane monoanion [closo-1-H-CB11Me11]-.

The synthesis of the silver(I) salt of the highly methylated carborane anion [closo-1-H-CB(11)Me(11)](-) is described, Ag[closo-1-H-CB(11)Me(11)] 1, which in the solid state shows close intermolecular Ag...H(3)C contacts. Addition of various monodentate phosphines to 1 results in the formation of the complexes (R(3)P)Ag[closo-1-H-CB(11)Me(11)] [R = Ph, 2; cyclohexyl (C(6)H(11)), 3; (3,5-Me(2)-C(6)H(3)), 4]. All these complexes show close intermolecular Ag.H(3)C contacts in the solid state that are considerably shorter than the sum of the van der Waals radius of methyl (2.00 A) and the ionic radius of silver(I) (1.29 A). For 2 and 3 there are other close intermolecular Ag...H(3)C contacts in the solid state, arising from proximate carborane anions in the crystal lattice. Addition of methyl groups to the periphery of the phosphine ligand (complex 4) switches off the majority of these interactions, leaving essentially a single cage interacting with the cationic silver-phosphine fragment through three CH(3) groups. In solution (CD(2)Cl(2)) Ag...H(3)C contacts remain, as evidenced by both the downfield chemical shift change and the significant line-broadening observed for the cage methyl signals. These studies also show that the metal fragment is fluxional over the surface of the cage. The Ag...H(3)C interactions in solution may be switched off by addition of a stronger Lewis base than [closo-1-H-CB(11)Me(11)](-). Thus, addition of [NBu(4)][closo-1-H-CB(11)H(5)Br(6)] to 2 affords (Ph(3)P)Ag[closo-1-H-CB(11)H(5)Br(6)], while adding Et(2)O or PPh(3) affords the well-separated ion-pairs [(Ph(3)P)(L)Ag][closo-1-H-CB(11)Me(11)] (L = OEt(2) 5, PPh(3) 6,) both of which have been crystallographically characterized. DFT calculations on 2 (at the B3LYP/DZVP level) show small energy differences between the possible coordination isomers of this compound, with the favored geometry being one in which the [(Ph(3)P)Ag](+) fragment interacts with three of the [BCH(3)] vertices on the lower surface of the cage, similar to the experimentally observed structure of 4.

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PubMed2003

Responses to exercise in the heat related to measures of hypothalamic serotonergic and dopaminergic function.

We have studied 12 recreationally active men to measure their responses to exercise in the heat and relate these to measures of hypothalamic function explored with a buspirone [5-hydroxytryptamine 1A (5-HT(1A)) agonist, dopaminergic D(2) antagonist] neuroendocrine challenge, with and without pretreatment with pindolol (5-HT(1A) antagonist). Pindolol treatment allowed the serotonergic and non-serotonergic components of prolactin release to be distinguished. Subjects exercised at 73 (5)% maximal rate of oxygen uptake (VO(2max)) until volitional fatigue at 35 degrees C (relative humidity, 30%). On another two occasions they underwent a buspirone challenge [0.5 mg (kg body mass)(-1)], once with, and once without, pindolol [0.5 mg (kg body mass)(-1)] pretreatment and the circulating plasma concentrations of prolactin were measured for the next 2.5 h. Rectal temperature increased throughout exercise, whilst mean skin temperature remained constant. There was a wide inter-subject variation in prolactin response to the neuroendocrine challenges. The proportion of the prolactin response to buspirone attributable to a non-serotonergic component (most likely dopaminergic) correlated both with exercise duration (r=0.657, P=0.028), rectal temperature at fatigue (r=0.623, P=0.041) and the rate of temperature rise (r=-0.669, P=0.024). Our results suggest that high activity of the dopaminergic pathways in the hypothalamus is a predictor of exercise tolerance in the heat.

Adaptation, Physiological↗
PubMed2002

Solution and solid-state structure of the anion [Ag(2)[closo-CB(11)H(12)](4)](2-).

Addition of the carbene 1,3-dimesitylimidazol-2-ylidene (IMes) to a toluene solution of Ag[closo-CB(11)H(12)] results in the formation of the complex [(IMes)(2)Ag](2)[Ag(2)[closo-CB(11)H(12)](4)], the anionic component of which contains two silver(I) centers bridged by two carboranes in addition to one terminally bound carborane on each metal, in the solid-state. Comparison of the observed (11)B[(1)H] NMR chemical shifts of [(IMes)(2)Ag](2)[Ag(2)[closo-CB(11)H(12)](4)] or Ag[closo-CB(11)H(12)] with [NBu(4)][closo-CB(11)H(12)] in CD(2)Cl(2) demonstrates that the silver ion interacts significantly with the cage in solution. Theoretical investigations using the ab initio/GIAO/NMR method of [closo-CB(11)H(12)](-) and Na[closo-CB(11)H(12)] as model geometries for the silver salts support experimental evidence for these Ag...[BH] interactions in solution.

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